1. Field of the Invention
The present invention relates to a balance transformer used in a circuit for parallel-driving a plurality of discharge lamps to balance current shunted to the plurality of discharge lamps, and in particular, to a balance transformer suitable for use in a DC/AC inverter circuit for parallel-driving cold cathode fluorescent lamps (CCFL) for back light of various types of display panels used for Laptops, liquid crystal televisions, and the like.
2. Description of the Prior Art
Heretofore, as a parallel-drive circuit of CCFL, for example, those disclosed in International Patent Publication No. WO2005/038828, U.S. Pat. No. 6,781,325, and Japanese Unexamined Patent Publication No. 2003-31383 are known.
Further, Japanese Unexamined Patent Publication No. 2006-12781 proposes a balance transformer disposed with a plurality of transformer portions comprising a primary winding and a secondary winding disposed coaxially applicable to a parallel drive circuit of the CCFL of this type, particularly as a balance transformer applicable to the parallel drive circuit disclosed in the International Patent Publication No. WO2005/038828.
The balance transformer disclosed in the above described Japanese Unexamined Patent Publication No. 2006-12781 can be broadly classified into three types in its specific mode. A first type is disposed with a plurality of transformer portions in series, and at the same time, is disposed with a loop-shaped common core communicated to the interior of the winding of each transformer (see FIGS. 11, 12, and 14 of the above described Japanese Unexamined Patent Publication No. 2006-12781). A second type is disposed with a plurality of transformer portions in parallel, and at the same time, is disposed with a common core provided with a plurality of leg portions inserted into the interior of the winding of each transformer, respectively (see FIG. 13 of the above described Japanese Unexamined Patent Publication No. 2006-12781). Further, a third type is disposed with an individual core for each of the plurality of transformer portions disposed in parallel (see FIG. 15 of the above described Japanese Unexamined Patent Publication No. 2006-12781).
However, the balance transformers of the first and second type are liable to cause magnetic interference since a magnetic path for each magnetic flux generated at each transformer portion is mutually not isolated, and there is a fear that an accuracy of the operation for balancing the current to each CCFL is lowered.
On the other hand, though each magnetic path is mutually isolated by providing a separate core for each transformer, the balance transformer of the third type is hardly able to attempt at the miniaturization of the individual transformer portion because of the requirement of a core for each transformer portion, and at the same time, the number of component parts of each transformer portion is increased, thereby causing a problem of the increase in the cost of production.